Image sensor

ABSTRACT

An image sensor includes: a photoelectric conversion unit that photoelectrically converts light to generate an electric charge; a holding unit that holds the electric charge generated by the photoelectric conversion unit; an accumulation unit that accumulates the electric charge generated by the photoelectric conversion unit; a first transfer path that transfers the electric charge generated by the photoelectric conversion unit to the accumulation unit; and a second transfer path that transfers the electric charge generated by the photoelectric conversion unit to the accumulation unit via the holding unit.

TECHNICAL FIELD

The present invention relates to an image sensor.

BACKGROUND ART

An image sensor driven in a global electronic shutter mode is known (seePTL1). With such an image sensor, it is difficult to perform signalreadout by a rolling electronic shutter mode.

CITATION LIST Patent Literature

PTL1: Japanese Laid-Open Patent Publication No. 2012-84644

SUMMARY OF INVENTION Solution to Problem

According to a first aspect of the present invention, an image sensorcomprises: a photoelectric conversion unit that photoelectricallyconverts light to generate an electric charge; a holding unit that holdsthe electric charge generated by the photoelectric conversion unit; anaccumulation unit that accumulates the electric charge generated by thephotoelectric conversion unit; a first transfer path that transfers theelectric charge generated by the photoelectric conversion unit to theaccumulation unit; and a second transfer path that transfers theelectric charge generated by the photoelectric conversion unit to theaccumulation unit via the holding unit.

According to a second aspect of the present invention, an image sensorcomprises: a photoelectric conversion unit that photoelectricallyconverts light to generate an electric charge; a holding unit that holdsthe electric charge generated by the photoelectric conversion unit; anaccumulation unit that accumulates the electric charge generated by thephotoelectric conversion unit; a first transfer unit that transfers theelectric charge generated by the photoelectric conversion unit to theaccumulation unit; and a second transfer unit that transfers theelectric charge held in the holding unit to the accumulation unit.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a configuration example of a digitalcamera.

FIG. 2 is a view illustrating an outline of an image sensor.

FIG. 3 is a circuit diagram illustrating a configuration of a pixel ofthe image sensor.

FIG. 4 is a view illustrating an arrangement of elements in the pixel.

FIG. 5 is a diagram illustrating an overall timing in a global shutteroperation.

FIG. 6 is a time chart illustrating various control signals supplied toeach pixel in a global shutter operation.

FIG. 7(a) and FIG. 7(b) are diagrams illustrating overall timings inother global shutter operations.

FIG. 8 is a time chart illustrating various control signals supplied toeach pixel in a global electronic shutter operation.

FIG. 9 is a diagram illustrating an overall timing in a rolling shutteroperation.

FIG. 10(a) is a time chart illustrating various control signals at thestart of an accumulation period in a first rolling shutter operation,and FIG. 10(b) is a time chart illustrating various control signals at atime of shifting from the accumulation period to a readout.

FIG. 11(a) is a time chart illustrating various control signals at thestart of an accumulation period in a second rolling shutter operation,and FIG. 11(b) is a time chart illustrating various control signals at atime of shifting from the accumulation period to a readout.

FIG. 12(a) is a time chart illustrating various control signals at thestart of an accumulation period in a third rolling shutter operation,and FIG. 12(b) is a time chart illustrating various control signals at atime of shifting from the accumulation period to a readout.

DESCRIPTION OF EMBODIMENTS

An image sensor according to the present embodiment is configured to becapable of performing an operation in a global electronic shutter mode(hereinafter, a global shutter operation) and an operation in a rollingelectronic shutter mode (hereinafter, referred to as a rolling shutteroperation). The global shutter operation refers to a driving mode ofperforming reset of photodiodes included in the image sensor, generationof electric charges in the photodiodes, and the like, simultaneously inall rows (i.e., all pixels). For the global shutter operation,accumulation processes are simultaneous for all rows because theaccumulation processes can be simultaneously started for all pixels.

On the other hand, the rolling shutter operation refers to a drivingmode of performing reset of photodiodes included in the image sensor,generation of electric charges in the photodiodes, and the like, row byrow. For the rolling shutter operation, accumulation processes are notsimultaneous for all rows because timings of starting the accumulationprocesses are different from one row to another, even when accumulationtimes (electric charge generation times) are the same for all rows.

Details will be described below with reference to the drawings.

FIG. 1 is a view schematically illustrating a configuration example of adigital camera including an image sensor 101 according to oneembodiment. The digital camera includes an interchangeable lens 110 anda camera body 100, and the interchangeable lens 110 is attached to thecamera body 100 via a lens mount unit 105.

Note that the digital camera may be a lens-integrated camera, instead ofa lens-interchangeable camera.

The interchangeable lens 110 includes, for example, a lens control unit111, a zoom lens 112, a focus lens 113, an anti-vibration lens 114, anaperture 115, a lens operation unit 116, and the like. The lens controlunit 111 includes a CPU and peripheral components such as a memory. Thelens control unit 111 performs drive control of the focus lens 113 andthe aperture 115, position detection of the zoom lens 112 and the focuslens 113, transmission of lens information to the camera body 100,reception of camera information from the camera body 100, and the like.

The camera body 100 includes, for example, the image sensor 101, a bodycontrol unit 102, a body operation unit 103, a display unit 104, and thelike. The image sensor 101 is arranged in a predetermined imaging plane(intended focus plane) of the interchangeable lens 110 tophotoelectrically convert a subject image formed by the interchangeablelens 110. The body operation unit 103 includes a shutter button,operation members for various settings, and the like. The display unit104 includes, for example, a liquid crystal monitor (also referred to asa rear monitor) mounted on a rear surface of the camera body 100.

The body control unit 102 includes a CPU and peripheral components suchas a memory. The body control unit 102 performs operation control of thedigital camera, such as drive control of the image sensor 101, readoutof image signals from the image sensor 101, focus detection calculationand focus adjustment of the interchangeable lens 110, and processing andrecording of image signals. Further, the body control unit 102 performscommunication with the lens control unit 111 via an electric contact 106provided in the lens mount unit 105 to receive lens information andtransmit camera information (a defocus amount, an aperture value, andthe like).

A light flux having passed through the interchangeable lens 110 forms asubject image on a light-receiving surface of the image sensor 101. Thesubject image is photoelectrically converted by the image sensor 101,and a signal resulted from the photoelectric conversion is thentransmitted to the body control unit 102.

The body control unit 102 detects a focus adjustment state (defocusamount) of the interchangeable lens 110 by performing a known focusdetection calculation based on the signal from the image sensor 101. Thedefocus amount detected by the body control unit 102 is transmitted tothe lens control unit 111.

The lens control unit 111 calculates a drive amount of the focus lens113 based on the received defocus amount. The lens control unit 111 thendrives a motor (not shown) or the like based on the calculated driveamount, to cause the focus lens 113 to be moved to a focus position.

Further, the body control unit 102 processes the signal from the imagesensor 101 to generate image data, and stores the image data in a memorycard (not shown). The body control unit 102 further causes the displayunit 104 to display a monitor image (also referred to as a live viewimage) based on the signal from the image sensor 101.

Configuration of Image Sensor

FIG. 2 is a schematic view illustrating an outline of the image sensor101. The image sensor 101 is a CMOS image sensor. The image sensor 101includes a pixel area 201, a vertical control unit 202, a horizontalcontrol unit 203, an output unit 204, and a control unit 205. Note thata power supply unit and a detailed circuit diagram are omitted in FIG.2.

The pixel area 201 has a plurality of pixels arranged two-dimensionallyin a horizontal direction (row direction) and a vertical direction(column direction), for example. Each pixel has a photodiode(photoelectric conversion unit) that generates an electric charge inaccordance with an incident light amount. Each of the plurality ofpixels is driven by the vertical control unit 202 and the horizontalcontrol unit 203, and a signal based on the electric charge generated inthe photodiode of each pixel is read out via a vertical signal line 210.

The output unit 204 performs a correlated double sampling (CDS) on thesignal read out from each pixel and applies a gain on the signal, asnecessary. The signal processed by the output unit 204 is output to asignal processing unit (not shown) located downstream.

In the above description, an example has been described in which theoutput unit 204 outputs signals as analog signals to the signalprocessing unit located downstream. However, the output unit 204 mayinclude an A/D converter to output signals after A/D conversion asdigital signals.

Further, an example is illustrated in which the output unit 204 outputssignals read out via the vertical signal lines 210 to the signalprocessing unit located downstream, in parallel. However, signals may beoutput to the signal processing unit located downstream, one by one,after a horizontal transfer in the output unit 204.

The control unit 205 controls the elements of the image sensor 101described above. That is, a global shutter operation and a rollingshutter operation of the image sensor 101 described below are performedunder the control of the control unit 205 in response to commands fromthe body control unit 102.

Note that in the present embodiment, a “pixel” includes a photodiode anda readout unit that reads out a signal based on an electric chargegenerated in the photodiode. An example will be illustrated in which thereadout unit includes transfer transistors, a floating diffusion (FD)region, an amplification transistor, and a selection transistor asdescribed later. However, the readout unit is not limited to the presentexample.

FIG. 3 is a circuit diagram illustrating a configuration of a unit pixel30 of the image sensor 101. In FIG. 3, each pixel 30 has a photodiodePD, a diffusion capacitor SC, six transistors (a diffusion capacitancetransfer transistor MEM, a first transfer transistor Tx1, a secondtransfer transistor Tx2, a reset transistor RST, an amplificationtransistor SF, and a selection transistor SEL), and a FD region. Theelements of the pixel 30 are connected to one another as shown in FIG.3. In FIG. 3, reference symbol VDD denotes a power supply voltage.

The diffusion capacitance transfer transistor MEM transfers an electriccharge generated in the photodiode PD to the diffusion capacitor SC. Thediffusion capacitance transfer transistor MEM is an electrode fortransferring the electric charge generated in the photodiode PD to thediffusion capacitor SC. The diffusion capacitance transfer transistorMEM is turned on to transfer the electric charge when a control signalφMEM becomes its high level, and turned off when the control signal φMEMbecomes its low level. The diffusion capacitor SC functions as anelectric charge holding unit that holds the electric charge transferredfrom the photodiode PD by the diffusion capacitance transfer transistorMEM.

The first transfer transistor Tx1 transfers the electric charge held inthe diffusion capacitor SC to the FD region. The first transfertransistor Tx1 is turned on to transfer the electric charge when acontrol signal φTx1 becomes its high level, and turned off when thecontrol signal φTx1 becomes its low level.

The FD region converts the transferred electric charge into a voltage.The amplification transistor SF forms a source follower circuit toamplify a signal in accordance with a potential of the FD region. Thereset transistor RST resets the electric charges in the FD region, thediffusion capacitor SC, and the photodiode PD. Details of the resetoperation will be described later.

The selection transistor SEL is a transistor for row selection, whichoutputs the signal amplified by the amplification transistor SF to acorresponding vertical signal line 210. The selection transistor SEL isturned on to output the signal when a control signal φSEL becomes itshigh level, and turned off when the control signal φSEL becomes its lowlevel.

The second transfer transistor Tx2 transfers the electric chargegenerated in the photodiode PD to the FD region. The second transfertransistor Tx2 is turned on to transfer the electric charge when acontrol signal φTx2 becomes its high level, and turned off when thecontrol signal φTx2 becomes its low level.

FIG. 4 is a view illustrating an arrangement of the elements in thepixel 30. The 2 5 diffusion capacitance transfer transistor MEM(diffusion capacitor SC) is arranged next to the photodiode PD. Thediffusion capacitance transfer transistor MEM (diffusion capacitor SC)is shielded from light. The FD region is connected to the diffusioncapacitance transfer transistor MEM (diffusion capacitor SC) via thefirst transfer transistor Tx1, and to the photodiode PD via the secondtransfer transistor Tx2.

The FD region is further connected to a control terminal of theamplification transistor SF, and to the power supply VDD via the resettransistor RST. An output terminal of the amplification transistor SF isconnected to the vertical signal line 210 via the selection transistorSEL.

As described above, the image sensor 101 can perform the global shutteroperation and the rolling shutter operation. Timings of the operationswill be described with reference to time charts. In the followingdescription, an accumulation time (electric charge generation time)corresponds to an exposure time determined by the body control unit 102based on a known exposure calculation or an exposure time determined bya user operating the body operation unit 103. A time length from thestart to the end of an accumulation period is the accumulation time.

Global Shutter Operation

FIG. 5 is a diagram illustrating an overall timing of the global shutteroperation. In FIG. 5, the vertical axis represents pixel rows providedin the pixel area 201 (FIG. 2) and the horizontal axis represents time.A time length from a time point t1 at which the photodiode PD is resetin a P-th frame to a time point t2 at which the electric chargegenerated in the photodiode PD is transferred to the diffusion capacitorSC corresponds to the accumulation time of the P-th frame. After theelectric charges is transferred from the photodiode PD to the diffusioncapacitor SC, the electric charge is read out from the diffusioncapacitor SC row by row. According to FIG. 5, in the global shutteroperation, the accumulation time from the PD reset time point (t1) tothe PD->SC transfer time point (t2) is simultaneous for every row. In areadout period, time differences occur among the rows because thereadout of the electric charge from the diffusion capacitor SC isperformed row by row.

A time length from a time point t1′ at which the photodiode PD is resetin a (P+1)-th frame, which is the next frame, to a time point t2′ atwhich the electric charge generated in the photodiode PD is transferredto the diffusion capacitor SC corresponds to the accumulation time ofthe (P+1)-th frame. After the electric charge is transferred from thephotodiode PD to the diffusion capacitor SC, the electric charge is readout from the diffusion capacitor SC row by row, as in the case of theP-th frame.

FIG. 6 is a time chart illustrating various control signals supplied toeach pixel 30 of the image sensor 101 in the global shutter operation.One frame period includes an accumulation period and a readout period.

1. Accumulation Period

The control unit 205 causes all the pixels 30 in the pixel area 201(FIG. 2) to simultaneously perform accumulation, and causes thephotodiode PD to be reset at the start of the accumulation period.

In FIG. 6, a high-level reset pulse is supplied to the reset transistorRST as a control signal φRST in accordance with an instruction from thecontrol unit 205. Thereby, the reset transistor RST is turned on so thata potential of the FD region is reset.

Subsequently, a high-level pulse is supplied to the second transfertransistor Tx2 as a control signal φTx2 in accordance with aninstruction from the control unit 205. Thereby, the second transfertransistor Tx2 is turned on so that unnecessary electric charge existingin the photodiode PD is discharged (PD resetting).

By providing the second transfer transistor Tx2 in the image sensor 101,the photodiode PD can be reset in a shorter time than in a case wherethe second transfer transistor Tx2 is not provided.

The photodiode PD after the reset generates and accumulates an electriccharge in accordance with an incident light amount.

During the accumulation, a high-level reset pulse is supplied to thereset transistor RST as a control signal φRST in accordance with aninstruction from the control unit 205. Thereby, the reset transistor RSTis turned on so that a potential of the FD region is reset.Subsequently, a high-level pulse is supplied to the first transfertransistor Tx1 as a control signal φTx1 in accordance with aninstruction from the control unit 205. As a result, the first transfertransistor Tx1 is turned on so that unnecessary electric charge existingin the diffusion capacitor SC is discharged (SC resetting).

After the diffusion capacitor SC is reset, a high-level pulse issupplied to the diffusion capacitance transfer transistor MEM as acontrol signal φMEM in accordance with an instruction from the controlunit 205. As a result, the diffusion capacitance transfer transistor MEMis turned on so that the electric charge of the photodiode PD istransferred to the diffusion capacitot SC. The accumulation period endswith electric charge transfer from the photodiode PD to the diffusioncapacitor SC.

2. Readout Period

The control unit 205 sequentially reads out the electric charge row byrow, from the diffusion capacitor SC of every pixel 30 in the pixel area201 (FIG. 2). In FIG. 6, a range enclosed by a dashed line is a readoutperiod. FIG. 6 shows only a waveform illustrating readout for one row,and the illustration of readout waveforms for other rows is omitted.However, the same readout is performed for each row.

In FIG. 6, a high-level control signal φSEL is supplied to the selectiontransistor SEL for a predetermined period of time in accordance with aninstruction from the control unit 205. This allows the selectiontransistor SEL to be turned on for a predetermined period. Further, ahigh-level reset pulse is supplied to the reset transistor RST as acontrol signal φRST. Thereby, the reset transistor RST is turned on sothat a potential of the FD region is reset. Then, at a time pointdenoted by a dashed-dotted line Dark, a reset level signal is read outby the control unit 205 via a corresponding vertical signal line 210.

Subsequently, a high-level transfer pulse is supplied to the firsttransfer transistor Tx1 as a control signal φTx1 in accordance with aninstruction from the control unit 205. As a result, the first transfertransistor Tx1 is turned on so that the electric charge of the diffusioncapacitor SC is transferred to the FD region. Then, at a time pointdenoted by a dashed-dotted line Sig, a signal-level signal is read outby the control unit 205 via a corresponding vertical signal line 210.

Other Operation Examples

In FIG. 5, the readout period is started only after the accumulationperiod for one frame ends and then the accumulation period for the nextframe is started only after the readout period ends.

Instead of the way in FIG. 5, the accumulation period for the next framemay be started without waiting for the end of the signal readout periodof the previous frame. FIG. 7(a) and FIG. 7(b) are diagrams illustratingoverall timings in other global shutter operations. For example, when acontinuous shooting mode of continuously photographing still images isset or when a moving image mode of capturing moving images is set, theglobal shutter operation may be performed based on the timing accordingto FIG. 7(a) or FIG. 7(b).

FIG. 7(a) is a diagram illustrating a case where the accumulation timeis greater than the readout time. The vertical axis indicates pixel rowsprovided in the pixel area 201 (FIG. 2) and the horizontal axisindicates time. According to FIG. 7(a), the accumulation of the (P+1)-thframe is started at a time point t1′ without waiting for the end of thesignal readout period of the P-th frame. That is, the accumulation ofthe (P+1)-th frame and the signal readout of the P-th frame areperformed in parallel. Thus, in FIG. 7(a), the signal readout period ofthe P-th frame is included in the accumulation period of the (P+1)-thframe.

FIG. 7(b) is a diagram illustrating a case where the accumulation timeis less than the readout time. The vertical axis indicates pixel rowsprovided in the pixel area 201 (FIG. 2) and the horizontal axisindicates time. In FIG. 7(b), the accumulation of the (P+1)-th frame isstarted without waiting for the end of the readout period of the signalof the P-th frame, as in the case of FIG. 7(a). However, theaccumulation of the (P+1)-th frame is started at a time point t1′, atwhich a time elapsed since the end time point t2 of the accumulationperiod of the P-th frame is longer than that in FIG. 7(a), so that thesignal readout period of the P-th frame ends before unnecessary electriccharge existing in the diffusion capacitor SC is discharged (SCresetting) in the accumulation period of the (P+1)-th frame. That is,the start of the accumulation of the (P+1)-th frame is later than thestart of the signal readout period f the P-th frame. In this case, thetime point t1′ is within the signal readout period of the previousframe. Thus, while discharging unnecessary electric charges existing inthe photodiode PD (PD resetting) at the start of the accumulation periodof the (P+1)-th frame, the signal readout of the P-th frame istemporarily interrupted and restarted after the PD resetting.

FIG. 8 is a time chart illustrating various control signals supplied toeach pixel 30 of the image sensor 101 in the global shutter operationcorresponding to FIG. 7(a). As in FIG. 6, a range enclosed by a dashedline is a readout period. FIG. 8 shows only a waveform illustratingreadout for one row, and the illustration of readout waveforms for otherrows is omitted. However, the same readout is performed for each row.

When comparing FIG. 8 with FIG. 6, before the readout for the P-th frameis started, the accumulation of the (P+1)-th frame is started andunnecessary electric charge existing in the photodiode PD is discharged(PD resetting).

Although a time chart in the global shutter operation corresponding toFIG. 7(b) is not shown, the PD resetting of the (P+1)-th frame in FIG. 8is performed within the readout period (in a range enclosed by a dashedline) for the P-th frame. Therefore, as described with reference to FIG.7(b), the signal readout of the P-th frame is temporarily interrupted,and the PD reset of the (P+1)-th frame is performed during theinterruption.

Rolling Shutter Operation

FIG. 9 is a diagram illustrating an overall timing in a rolling shutteroperation. In FIG. 9, the vertical axis represents pixel rows providedin the pixel area 201 (FIG. 2) and the horizontal axis represents time.For example, a time length from a time point t1 at which the photodiodePD is reset in a first row of the P-th frame to a time point t1 e atwhich the electric charge generated in the photodiode PD in the firstrow is transferred to the FD region corresponds to the accumulation timeof the first row. After the electric charge is transferred from thephotodiode PD to the FD region, a signal based on the electric charge isread out. The above-described operation is performed for each row. Atime length from a time point tN at which the photodiode PD is reset ina N-th row to a time point tNe at which the electric charge generated inthe photodiode PD in the N-th row is transferred to the FD regioncorresponds to the accumulation time of the N-th row.

According to FIG. 9, in the rolling shutter operation, accumulations forthe rows are not simultaneous and time differences occur among the rows,even though accumulation periods of time are the same for the first rowto the N-th row. The same applies to the (P+1)-th frame, which is thenext frame, and subsequent frames.

In the present embodiment, three types of rolling shutter operations areavailable. A first rolling operation is an operation in which, when theelectric charge of the photodiode PD is saturated, an overflowedelectric charge is accumulated in the diffusion capacitor SC and thenthe electric charge of the photodiode PD and the electric charge of thediffusion capacitor SC are simultaneously transferred to the FD region.This is suitable for a case where the FD region has an enoughcapacitance so that a dynamic range can be increased. Note that thecapacitance of the FD region is determined when the image sensor 101 isdesigned.

A second rolling operation is an operation of transferring the electriccharge of the photodiode PD to the diffusion capacitor SC and thentransferring the electric charge from the diffusion capacitor SC to theFD region. Even if the FD region has not enough capacitance, a dynamicrange can be increased.

A third rolling operation is an operation of directly transferring theelectric charge of the photodiode PD to the FD region without passingthrough the diffusion capacitance transfer transistor MEM (diffusioncapacitance SC). For any of the rolling shutter operations, the overalltiming is as described in FIG. 9.

First Rolling Operation

FIG. 10(a) and FIG. 10(b) are time charts illustrating various controlsignals supplied to each pixel 30 of the image sensor 101 in the firstrolling shutter operation. FIG. 10(a) is a time chart illustratingvarious control signals at the start of the accumulation period, andFIG. 10(b) is a time chart illustrating various control signals at atime of shifting from the accumulation period to a readout.

In FIG. 10(a), the control unit 205 sequentially performs accumulationfor the pixel rows in the pixel area 201 (FIG. 2), and resets thephotodiode PD and the diffusion capacitor SC at the start of theaccumulation period. Specifically, a high-level reset pulse is suppliedto the reset transistor RST as a control signal φRST in accordance withan instruction from the control unit 205. Thereby, the reset transistorRST is turned on so that a potential of the FD region is reset.

Subsequently, high-level pulses are supplied to the first transfertransistor Tx1 and the second transfer transistor Tx2 as a controlsignal φTx1 and a control signal φTx2, respectively, in accordance withan instruction from the control unit 205. When the high-level pulse issupplied to the second transfer transistor Tx2, the second transfertransistor Tx2 is turned on so that unnecessary electric charge existingin the photodiode PD is discharged (PD resetting). Further, a high-levelpulse is supplied to the first transfer transistor Tx1, and thus thefirst transfer transistor Tx1 is turned on so that unnecessary electriccharge existing in the diffusion capacitor SC is discharged (SCresetting).

Further, in accordance with an instruction from the control unit 205, acontrol signal φMEM is controlled so that a voltage of the electrode ofthe diffusion capacitance transfer transistor MEM is set to anintermediate voltage. Setting the voltage of the electrode of thediffusion capacitance transfer transistor MEM to the intermediatevoltage is intended for the electric charge to overflow from thephotodiode PD into the diffusion capacitor SC. That is, the photodiodePD after the reset generates and accumulates an electric charge inaccordance with an incident light amount and then, when the photodiodePD is saturated and the electric charge overflows, the overflowedelectric charge is accumulated in the diffusion capacitor SC. A dynamicrange can be increased because electric charges are accumulated in thephotodiode PD and the diffusion capacitor SC.

In FIG. 10(b), the control unit 205 resets the FD region at the end ofthe accumulation period. Specifically, a high-level reset pulse issupplied to the reset transistor RST as a control signal φRST inaccordance with an instruction from the control unit 205. Thereby, thereset transistor RST is turned on so that a potential of the FD regionis reset. Then, at a time point denoted by a dashed-dotted line Dark,the reset level signal is read out by the control unit 205 via acorresponding vertical signal line 210.

In the first rolling shutter operation, the generated electric chargesare transferred from the photodiode PD and the diffusion capacitor SC tothe FD region at one time. Specifically, high-level pulses are suppliedto the first transfer transistor Tx1 and the second transfer transistorTx2 as a control signal φTx1 and a control signal φTx2, respectively, inaccordance with an instruction from the control unit 205. As a result,the first transfer transistor Tx1 is turned on so that the electriccharge accumulated in the diffusion capacitor SC is transferred to theFD region. In parallel thereto, the second transfer transistor Tx2 isturned on so that the electric charge accumulated in the photodiode PDis transferred to the FD region. The accumulation period ends withelectric charge transfer to the FD region. Then, at a time point denotedby a dashed-dotted line Sig, the signal level signal is read out by thecontrol unit 205 via a corresponding vertical signal line 210.

Second Rolling Operation

FIG. 11(a) and FIG. 11(b) are time charts illustrating various controlsignals supplied to each pixel 30 of the image sensor 101 in the secondrolling shutter operation. FIG. 11(a) is a time chart illustratingvarious control signals at the start of the accumulation period, andFIG. 11(b) is a time chart illustrating various control signals at atime of shifting from the accumulation period to a readout.

In FIG. 11(a), the control unit 205 sequentially performs accumulationfor the pixel rows in the pixel area 201 (FIG. 2), and resets thephotodiode PD and the diffusion capacitor SC at the start of theaccumulation period. Waveforms of the control signal φRST and thecontrol signals φTx1 and φTx2 are the same as those in the case of FIG.10(a). FIG. 11(a) is different from FIG. 10(a) in that the voltage ofthe electrode of the diffusion capacitance transfer transistor MEM isnot set to an intermediate voltage. With this configuration, theelectric charge does not overflow to the diffusion capacitor SC, evenwhen the photodiode PD, which generates and accumulates an electriccharge in accordance with an incident light amount after the reset, issaturated.

In FIG. 11(b), the control unit 205 resets the FD region at the end ofthe accumulation period. A waveform of the control signal φRST is thesame as that in the case of FIG. 10(b). At a time point denoted by adashed-dotted line Dark, the reset level signal is read out by thecontrol unit 205 via a corresponding vertical signal line 210.

In the second rolling shutter operation, the generated electric chargeis transferred from the photodiode PD to the diffusion capacitor SC, andfurther from the diffusion capacitor SC to the FD region in a relayform. Thus, differences from FIG. 10(b) are in that the diffusioncapacitance transfer transistor MEM is turned on before the firsttransfer transistor Tx1 is turned on and that the second transfertransistor Tx2 is not turned on.

When a high-level pulse is supplied to the diffusion capacitancetransfer transistor MEM as a control signal φMEM in accordance with aninstruction from the control unit 205, the diffusion capacitancetransfer transistor MEM is turned on so that the electric chargeaccumulated in the photodiode PD is transferred to the diffusioncapacitor SC. The accumulation period ends with electric charge transferfrom the photodiode PD to the diffusion capacitor SC. Thereafter, when ahigh-level transfer pulse is supplied to the first transfer transistorTx1 as a control signal φTx1, the first transfer transistor Tx1 isturned on so that the electric charge accumulated in the diffusioncapacitor SC is transferred to the FD region. Then, at a time pointdenoted by a dashed-dotted line Sig, the signal level signal is read outby the control unit 205 via a corresponding vertical signal line 210.

Third Rolling Operation

FIG. 12(a) and FIG. 12(b) are time charts illustrating various controlsignals supplied to each pixel 30 of the image sensor 101 in the thirdrolling shutter operation. FIG. 12(a) is a time chart illustratingvarious control signals at the start of the accumulation period, andFIG. 12(b) is a time chart illustrating various control signals at atime of shifting from the accumulation period to a readout.

In FIG. 12(a), the control unit 205 sequentially performs accumulationfor the pixel rows in the pixel area 201 (FIG. 2), and resets thephotodiode PD and the diffusion capacitor SC at the start of theaccumulation period. Waveforms of the control signal φRST and thecontrol signals φTx1 and φTx2 are the same as those in the case of FIG.11(a).

In FIG. 12(b), the control unit 205 resets the FD region at the end ofthe accumulation period. A waveform of the control signal φRST is thesame as that in the case of FIG. 11(b). At a time point denoted by adashed-dotted line Dark, the reset level signal is read out by thecontrol unit 205 via a corresponding vertical signal line 210.

In the third rolling shutter operation, the generated electric charge isdirectly transferred from the photodiode PD to the FD region withoutusing the diffusion capacitance transfer transistor MEM (diffusioncapacitor SC). Thus, differences from FIG. 11(b) are in that the firsttransfer transistor Tx1 and the diffusion capacitance transfertransistor MEM are not turned on and that the second transfer transistorTx2 is turned on.

When a high-level transfer pulse is supplied to the second transfertransistor Tx2 as a control signal φTx2 in accordance with aninstruction from the control unit 205, the second transfer transistorTx2 is turned on so that the electric charge accumulated in thephotodiode PD is transferred to the FD region. The accumulation periodends with electric charge transfer from the photodiode PD to the FDregion. Then, at a time point denoted by a dashed-dotted line Sig, thesignal level signal is read out by the control unit 205 via acorresponding vertical signal line 210.

The body control unit 102 determines whether the above-described imagesensor 101 is to perform the global shutter operation or the rollingshutter operation depending on an established mode. For example, thebody control unit 102 causes the image sensor 101 to perform the globalshutter operation when a single shooting mode is set for photographing astill image at a time of depression of a shutter button, or a continuousshooting mode is set for continuously photographing still images duringdepression of the shutter button. This is because a so-called rollingshutter distortion (a moving subject looks distorted in an image) ismore noticeable when the rolling shutter operation is performed inphotographing still images.

On the other hand, when the moving image mode is set, the body controlunit 102 causes the image sensor 101 to perform the rolling shutteroperation.

Note that the body control unit 102 may cause the image sensor 101 toperform the global shutter operation, even when the moving image mode ofphotographing moving images is set.

For example, when the body control unit 102 is set to a wide dynamicrange (HDR) mode in which subjects, from a high-luminance subject to alow-luminance subject, are photographed with rich gradations, the bodycontrol unit 102 causes the image sensor 101 to perform the firstrolling shutter operation or the second rolling shutter operation.

The first rolling shutter operation and the second rolling shutteroperation may be appropriately selected.

According to the Above-Described Embodiment, the Following OperationalAdvantages can be achieved.

(1) Each of a plurality of pixels 30 included in an image sensor 101includes: a photodiode PD that performs photoelectric conversion oflight to generate an electric charge; an FD region that accumulates theelectric charge generated in the photodiode PD; a diffusion capacitor SCthat holds the electric charge generated in the photodiode PD; a firsttransfer path (second transfer transistor Tx2) that transfers theelectric charge from the photodiode PD to the FD region; and a secondtransfer path (first transfer transistor Tx1) that transfers theelectric charge from the photodiode PD to the FD region via thediffusion capacitor SC. With this configuration, the electric chargegenerated in the photodiode PD can be transferred from the photodiode PDto the FD region via two transfer routes. This allows the global shutteroperation or the rolling shutter operation to be appropriatelyperformed, for example.

(2) Because the first transfer path (the second transfer transistor Tx2)of the image sensor 101 also serves as a path of discharging theelectric charge from the photodiode PD, the electric charge can bedischarged (reset) from the photodiode PD in a shorter time than in acase where the image sensor 101 has only the second transfer path thattransfers the electric charge from the photodiode PD to the FD regionvia the diffusion capacitor SC.

(3) The plurality of pixels 30 of the image sensor 101 are arranged in arow direction and a column direction, and controlled to perform a globalshutter operation in which the electric charges are dischargedsimultaneously in the plurality of rows, or a rolling shutter operationin which the electric charges are discharged row by row in the pluralityof rows, so that the two electronic shutter operations can beappropriately performed.

(4) In a case where the image sensor 101 performs the global shutteroperation, when the second transfer path (the first transfer transistorTx1) is selected, the electric charges generated in the photodiodes PDare simultaneously transferred to the diffusion capacitors SC in thepixels 30 in a plurality of rows and then the electric chargestransferred to the diffusion capacitors SC are transferred to the FDregions row by row. With this configuration, the global shutteroperation can be appropriately performed.

(5) In a case where the image sensor 101 performs the global shutteroperation, when the electric charge generated in the photodiode PD istransferred to the diffusion capacitor SC in each of the pixels 30 inthe plurality of rows, the electric charge is discharged in the pixels30 in the plurality of rows before starting transfer from the diffusioncapacitor SC to the FD region. Therefore, a photoelectric conversion forthe next frame can be started earlier.

(6) In a case where the image sensor 101 performs the first rollingshutter operation, in the pixels 30 in the plurality of rows, when thefirst transfer path (the second transfer transistor Tx2) and the secondtransfer path (the first transfer transistor Tx2) are selected, theelectric charge generated in the photodiode PD is transferred to the FDregion through the first transfer path, and then to the FD regionaccumulation unit through the second transfer path via the diffusioncapacitor SC, for each row. With this configuration, in the firstrolling shutter operation, the electric charge can be efficientlytransferred in parallel via two routes.

(7) In performing the first rolling shutter operation, the diffusioncapacitor SC of the image sensor 101 is configured to hold an electriccharge overflowing from the photodiode PD, so that a dynamic range canbe increased compared with a case where the electric charge overflowingfrom the photodiode PD is wastefully discarded.

(8) In a case where the image sensor 101 performs the second rollingshutter operation, when the second transfer path (the first transfertransistor Tx1) is selected, the electric charge generated in thephotodiode PD of each of the plurality of pixels 30 is transferred tothe diffusion capacitor SC row by row, and then the electric chargetransferred to the diffusion capacitor SC is transferred to the FDregion. With this configuration, a dynamic range can be increased ascompared with a case where the transfer is performed without passingthrough the diffusion capacitor SC.

(9) In a case where the image sensor 101 performs the third rollingshutter operation, when the first transfer path (the second transfertransistor Tx2) is selected, the electric charge generated in thephotodiode PD of each of the plurality of pixels 30 is transferred tothe FD region row by row. With this configuration, the rolling shutteroperation can be performed in a mode different from the above (6) to(8).

In the above description, an example has been described in which theimage sensor 101 is mounted in a digital camera. However, the imagesensor 101 may be mounted in an electronic device other than the digitalcamera, such as a smartphone, a tablet terminal, or a wearable terminal.

The present invention is not limited to the above-described embodiment.Other embodiments that can be contemplated within the scope of thetechnical concept of the present invention are also included in thescope of the present invention.

The disclosure of the following priority application is hereinincorporated by reference:

Japanese Patent Application No. 2017-192167 (filed on Sep. 29, 2017)

REFERENCE SIGNS LIST

-   30 . . . pixel-   100 . . . camera body-   101 . . . image sensor-   102 . . . body control unit-   201 . . . pixel area-   202 . . . vertical control unit-   203 . . . horizontal control unit-   204 . . . output unit-   205 . . . control unit-   210 . . . vertical signal line-   MEM . . . diffusion capacitance transfer transistor-   PD photodiode-   SC . . . diffusion capacitor-   SF . . . amplification transistor-   Tx1 . . . first transfer transistor-   Tx2 . . . second transfer transistor

1. An image sensor comprising: a photoelectric conversion unit thatphotoelectrically converts light to generate an electric charge; aholding unit that holds the electric charge generated by thephotoelectric conversion unit; an accumulation unit that accumulates theelectric charge generated by the photoelectric conversion unit; a firsttransfer path that transfers the electric charge generated by thephotoelectric conversion unit to the accumulation unit; and a secondtransfer path that transfers the electric charge generated by thephotoelectric conversion unit to the accumulation unit via the holdingunit.
 2. The image sensor according to claim 1, wherein: at least one ofthe first transfer path and the second transfer path serves as a paththat discharges the electric charge from the photoelectric conversionunit.
 3. The image sensor according to claim 1, wherein: a plurality ofpixels are arranged in a first direction and in a second directionintersecting the first direction; and the image sensor comprises: acontroller that performs at least one of a first control in which theelectric charges are simultaneously discharged in the plurality ofpixels arranged in the first direction and the second direction, and asecond control in which the electric charges are discharged in a set ofpixels arranged in the first direction.
 4. The image sensor according toclaim 1 wherein: a plurality of pixels are arranged in a first directionand in a second direction intersecting the first direction; and theimage sensor comprises: a controller that performs at least one of afirst control in which the electric charge is simultaneously transferredfrom the photoelectric conversion unit to the holding unit via thesecond transfer path in each of the plurality of pixels arranged in thefirst direction and the second direction, and a second control in whichthe electric charge is transferred from the photoelectric conversionunit to the accumulation unit via the first transfer path in each of aset of pixels arranged in the first direction.
 5. The image sensoraccording to claim 3, wherein: when the controller performs the firstcontrol, the second transfer path transfers the electric chargegenerated by the photoelectric conversion unit to the holding unit ineach of the plurality of pixels arranged in the first direction and thesecond direction, and the second transfer path transfers the electriccharge transferred to the holding unit, to the accumulation unit in eachset of pixels arranged in the first direction.
 6. The image sensoraccording to claim 5, wherein: when the electric charge generated by thephotoelectric conversion unit is transferred to the holding unit in eachof the set of pixels arranged in the first direction, the electriccharge is discharged in each of the set of pixels arranged in the firstdirection before transfer of the electric charge from the holding unitto the accumulation unit is started.
 7. The image sensor according toclaim 3 wherein: when the controller performs the second control, thefirst transfer path transfers the electric charge generated by thephotoelectric conversion unit to the accumulation unit for each set ofpixels arranged in the first direction, in the set of pixels arranged inthe first direction, and the second transfer path transfers the electriccharge generated by the photoelectric conversion unit to theaccumulation unit via the holding unit for each set of pixels arrangedin the first direction, in the set of pixels arranged in the firstdirection.
 8. The image sensor according to claim 7, wherein: theholding unit holds an electric charge overflowing from the photoelectricconversion unit.
 9. The image sensor according to claim 3, wherein: whenthe controller performs the second control, the second transfer pathtransfers the electric charge generated by the photoelectric conversionunit of each of the pixels to the holding unit and transfers theelectric charge transferred to the holding unit, to the accumulationunit, for each set of pixels arranged in the first direction.
 10. Theimage sensor according to claim 3, wherein: when the controller performsthe second control, the first transfer path transfers the electriccharge generated by the photoelectric conversion unit of each of thepixels to the accumulation unit, for each set of pixels arranged in thefirst direction.
 11. The image sensor according to claim 3, wherein: theaccumulation unit is a diffusion layer.
 12. The image sensor accordingto any claim 3, wherein: the first control is a control in a globalelectronic shutter mode, and the second control is a control in arolling shutter mode.
 13. An image sensor comprising: a photoelectricconversion unit that photoelectrically converts light to generate anelectric charge; a holding unit that holds the electric charge generatedby the photoelectric conversion unit; an accumulation unit thataccumulates the electric charge generated by the photoelectricconversion unit; a first transfer unit that transfers the electriccharge generated by the photoelectric conversion unit to theaccumulation unit; and a second transfer unit that transfers theelectric charge held in the holding unit to the accumulation unit.